In the oil and gas industry, following drilling of a vertical or horizontal wellbore into a formation for the production of oil or gas therefrom, the wellbore is typically cased and cemented to line the length of the wellbore. Lining the wellbore ensures safe control of production of fluids therefrom, prevents water from entering the wellbore and keeps the formation from “sloughing” or “bridging” into the wellbore.
It is well known that during the running in of a tubing string, such as a casing string, and particularly production casing, the casing may encounter tight spots and obstructions in the openhole wellbore, such as that created by sloughing of the wellbore wall into the open hole or as a result of the casing pushing debris ahead of the bottom end of the casing along the openhole until it forms a bridge. Such obstructions prevent the advance of the casing and require the openhole to be cleared in order to advance the casing to the bottom of the hole. This is particularly problematic in horizontal wellbores.
Should the casing string become sufficiently engaged in a mud pack formed at the obstruction, differential sticking may also occur, making advancing or removal of the casing from the wellbore extremely difficult.
While casing strings have been rotated to assist with moving past or through an obstruction, high torque created by trying to rotate a long string of casing may result in significant damage to the threads between casing joints and may cause centralizers and the like to drag and ream into the wellbore. While rotation of casing may be a viable option in a vertical wellbore, albeit fraught with problems, it is extremely difficult, if not impossible in a horizontal wellbore.
One option for addressing obstructions is to employ a washing technique; generally pumping fluids through the casing while the casing is axially reciprocated uphole and downhole. Should the washing technique be unsuccessful, it is known to trip out the casing and run in a mud motor on a drill string to drill out or ream the obstruction from the wellbore. Such repeated running in and tripping out of tubulars is time consuming, labor intensive and, as a result, very expensive.
Further, it has been contemplated to attach costly apparatus, such as mud motors, jetting or reaming tools, to the bottom of the casing string, however the apparatus is not retrievable thereafter from the wellbore and adds significantly to the cost of the casing operation.
Alternatively, others have contemplated providing teeth on the bottom of the casing string, or on a shoe at the bottom of the casing string, to assist with cutting away the obstruction as the casing is advanced during running in. Typically, the casing is also reciprocated or stroked during the clearing operation, or in some cases stroked at the same time as the casing is rotated. An example of a known reciprocating tool is a reaming tool, disclosed in U.S. Pat. No. 8,191,655 which issued on Jun. 5, 2012 to Halliburton Energy Services Inc.
Applicant, in related U.S. Pat. No. 8,973,682, incorporated by reference herein in its entirety, sets forth an obstruction-clearing tool that relies on reciprocation of the casing string to clear away wellbore obstructions. The tool uses an axially-actuated helical drive for rotational-actuation of a portion of the clearing tool, without the need to rotate the conveying casing string. Typically, a sleeve is axially extendable relative to a non-rotating mandrel during reciprocation of the mandrel and the casing string and is, at the same time, rotated about the mandrel as a result of the helical drive.
In embodiments, including those disclosed in Halliburton's U.S. Pat. No. 8,191,655, an axially-actuated helical drive can further comprise biasing means, such as a spring, which acts between the rotating and non-rotating components for biased-assistance in the extension of axially compressed components of the apparatus, such as when changing from a retracted position to an extended position.
In downhole operations, circumstances are known where an operator abandons a tool downhole and permanently cements the tool therein. Later, if the wellbore needs to be extended, a secondary drill string is run downhole through the casing string to drill or mill out the cemented tool in an operation called a drill-out operation. Generally, the drillable portions are made of less competent materials, such as aluminum and aluminum composites, cast iron or brass which facilitate being drilled out. In such cases, the portions that are made drillable are generally internal components which would otherwise interfere with or retard passage of the secondary drill string therethrough. The drill bit can also be drillable or its design accommodates passage of a drill string therethrough, such as a tubular drill bit.
In the case where the tool comprises a biasing member, such as the spring, it is inherent in its function that the spring is resistant to drilling, both as a result of the material from which the spring is constructed and the coiled configuration. If, during the drill-out operation, the biasing spring is inadvertently engaged by the drill, such as could readily occur as a result of the design of the Halliburton tool described in U.S. Pat. No. 8,191,655, the operation may be impeded, or possibly defeated, causing considerable problems with drilling through the cemented tool.
Ideally, what is required is a relatively simple and inexpensive apparatus for incorporation into the casing string for clearing wellbore obstructions, without the need for rotating the casing string, which can achieve an optimum extension of components within the tool from the retracted, axially compressed components therein. Further, it is desirable that the apparatus be drillable without a significant increase in operational costs.